1. Field of the Invention
This invention relates to an apparatus and method for laser machine ablation to produce complex patterns suitable for the rapid and accurate patterning of one or more layers of material while not etching an underlying different material layer.
2. Description of Related Art
The use of ablation patterning of various materials including polymers is well known. U.S. Pat. No. 4,508,749, Brannon et al., for example, discloses the use of ultraviolet radiation for etching through a polyimide layer. This patent is aimed at producing tapered openings through the polyimide layer to expose an underlying metal layer so that an electrical connection can be made to the metal. U.S. Pat. No. 5,236,551, issued for Pan describes the use of ablation etching to pattern a polymetric material layer that is then used as a mask for etching patterns in an underlying metal layer with chemical etchants.
Polymeric materials have high absorptivity in the region, UV and limited thermal diffusivity, limiting spread of absorbed energy, and facilitating energy density build up in the desired volume of material, and explosion of the material when threshold is exceeded. The ablation process stops automatically when the low absorptive, high thermal diffusivity metallic layer is exposed. The above patents do not disclose the desirable ablation etching of low UV absorptive dielectric materials used in the manufacture of semiconductor devices, e.g. silicon dioxide and silicon nitride, which may also be used in combination with polyimides.
Ablation of low absorptive dielectric materials requires radiant energies approaching those needed to ablate metals, which in turn limits ablation process selectivity. This is the problem Mitwalsky et al. solved in U.S. Pat. No. 5,843,363 which describes ablation etching of a pattern through one or more dielectric layers overlying a conductive material to provide electrical contact access through aligned openings. The principle disclosed to terminate ablation at successive layers involves end point detection including monitoring material specific emission, changes in surface reflectivity, and material ions. Other methods described include predictive end points using a specific number of laser energy pulses, changing the absorptive characteristics of the dielectric material by changing the radiation wavelength, and darkening the material to increase absorption.
A limitation of the prior art inventions described above is the use of an electrically conductive metallic underlying layer and the use of physical masks to define ablation etch patterns. In many patterning processes the underlying base material is not metallic but the need to terminate ablation etching at intermediate layers of different materials is needed. Further, the creation of physical masks is time consuming and costly. Thus, there is a clear need for an economical, programmable system to rapidly create and change ablation patterns. There is also a need for a means to accomplish end point layer detection including time, layer thickness, reflectivity and quantity of material removal. An ability to flexibly arrange alternate material layers and to create arbitrary ablation patterns could significantly reduce process time and cost.
Briefly described, the invention comprises an apparatus and method for laser machine ablation to produce complex patterns suitable for the rapid and accurate patterning of one or more layers of material while not etching an underlying different material layer. The invention combines the functions of a programmable thin film micromirror array to produce a maskless pattern for material ablation and a monitoring system to rapidly correct workpiece positional errors and accurately control material removal.
The preprogrammed thin film micromirror array (TMA) has individually addressable and moveable mirrors capable of redistributing the laser output beam energy to produce a desired two-dimensional machining pattern. Simple and complex predetermined energy patterns can be created and rapidly changed. Different patterns can be generated on successive laser energy pulses both in energy distribution and geometric location, to create accurate, complex three-dimensional machining of a workpiece, not easily achieved by conventional machining techniques. An electronic tracking system is included to precisely align the laser energy patterns with workpiece features/indices and to continuously monitor the removal of material from the workpiece.